Tidy up for 0.9.3: Shagu overlap warning, readme rewrite

- Warn once in chat when ShaguTweaks Debuff Timer draws on target debuffs.
- Use STANDARD_TEXT_FONT for timers and stack counts.
- Clean up comments and dead code; add .gitattributes and .gitignore.
- Rewrite README (ClassicAPI version table, ShaguTweaks compatibility);
  move history to CHANGELOG.md.
- Tests: 19 scenarios, no machine-specific paths.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_019W3rwutyoLNHxXPycYWSMc

tests/test_addon.py

@@ -1,7 +1,6 @@
 import sys
 from pathlib import Path
 import xml.etree.ElementTree as ET
-sys.path.insert(0, str(Path("work/pythonlibs").resolve()))
 from lupa.lua51 import LuaRuntime
 
 addon = Path(__file__).resolve().parents[1]
@@ -214,6 +213,19 @@ SlashCmdList.TARGETFRAMEBUFF("sweeps off"); assert(cd.cleared and not cd:IsShown
 SlashCmdList.TARGETFRAMEBUFF("sweeps on"); cd.cleared=false
 auras.HELPFUL={}; Tick(); assert(cd.cleared and not cd:IsShown())
 ''')
+run("ShaguTweaks Debuff Timer overlap warns once, only with ClassicAPI", 'InstallAPI(true); auras.HELPFUL={Aura("A")}', '''
+Tick(); assert(#messages==0)
+TargetFrameDebuff1.cd={readable={}}
+Tick(); Tick()
+local n=0
+for _,m in ipairs(messages) do if string.find(m,"Debuff Timer",1,true) then n=n+1 end end
+assert(n==1)
+''')
+run("no warning on the native path", 'nativeBuffs={"Native"}; nativeDebuffs={}', '''
+TargetFrameDebuff1.cd={readable={}}
+Tick(); Tick()
+for _,m in ipairs(messages) do assert(not string.find(m,"Debuff Timer",1,true)) end
+''')
 ET.parse(addon/"TargetFrameBuff.xml")
 print("PASS XML parsing")
 print(f"{passed} Lua regression scenarios passed; XML parsed.")